Magnetic recording media, such as magnetic tapes, magnetic sheets and magnetic disks, are widely used in the fields of audio, video and computers. Referring to magnetic tapes used in a video field, when recording or reproducing an image with a magnetic tape loaded in a cassette, the cassette is mounted on a video deck and the tape is scanned by a magnetic head while traveling along guide poles and rollers. Traveling and rewinding of the tape for recording and reproducing have to be stable and constant in repeated use. Usually, the tape travels at a high speed and contacts guide poles etc. Therefore, the surface of the tape, which is usually made of a material with high electrical resistance, has to provide the properties suitable for high speed traveling including an anti-electrostatic effect.
In addition, magnetic layer surface is finished smooth so that contact of a magnetic tape with a magnetic head is kept constant for gaining higher sensitivity, especially higher output at a high frequency range. However, extremely smooth magnetic layer surface is liable to cause problems in tape traveling due to increasing dynamic friction resistance.
Furthermore, in traveling in a video deck, a back side of a magnetic tape as well as a right side contacts the guide poles and rolls and therefore, the back side with poor traveling property and durability will result in having excessive tension exerted on the traveling tape and thus having the magnetic layer excessively rubbed by a magnetic head, which in turn causes not only magnetic layer damages and magnetic powder dropping from the magnetic layer, but also fluctuation of tape winding tension intensity and leads to irregular edges of a wound tape and then to irregular tape traveling in reproducing. These faults will degrade image or electromagnetic properties such as skew, jitter and S/N.
To solve these problems, it has been suggested that a back coating layer is formed on a back side of a magnetic tape.
As an example, a magnetic tape having on a back side a back coating layer containing inorganic powder can reduce contact area with guide poles etc. by irregular surface of the back coating layer, whereby friction coefficient is lowered. For example, Japanese Patent Publication Open to Public Inspection Nos. 130234/1982, 161135/1983, 53825/1982 and 2415/1983 disclose the examples of the back coating layers incorporating inorganic powder, and most of these examples specify particle sizes.
The back coating layers incorporating carbon black in place of inorganic powder are described in Japanese Patent Examined Publication No. 17401/1977. The back coating layers containing carbon black aim at obtaining an anti-electrifying effect attributable to electroconductivity of carbon black, a light-shielding effect and an irregularity effect. In this case, however, carbon black having a average particle size of 10 to 20 m.mu. is extremely poor in dispersibility in paint due to flocculated particles; therefore, the back coating layer prepared by such coating solution is likely to form thereon excessive irregularities caused by flocculated particles and drop out due to rather low adhesion of these flocculated particles with a binder, and such excessive irregularities tend to be transferred onto a magnetic layer when the tape is wound.
As stated above, carbon black with fine particle size is difficult to disperse equally so as to form uniform irregularities on a back coating layer, and is liable to form irregularities on a magnetic layer due to excessive irregularities on the back coating layer.
It has also been attempted to use carbon black with a relatively small average primary particle size of 10 to 60 .mu.m, and with an another average primary particle size of not less than 100 .mu.m in combination to reduce a friction resistance of a back coating layer (e.g. Japanese Patent Publication Open to Public Inspection Nos. 45938/1985, 45939/1985, 25023/1985, 38725/1985, 107729/1985, 185027/1984, 223937/1984, 111828/1982, 147308/1975). However, no sufficient improvement of traveling property and durability can be achieved by such combination use based simply on different particle sizes of carbon black.